Digital level sensing probe system

ABSTRACT

Digital level sensing probe system including digital probe constructed of two (2) concentric cylinders, forming a set of discrete, cylindrical capacitors that use the substance whose level is to be measured as a dielectric material. The set of capacitors is arranged along the axis of measurement where each capacitor represents a discrete level increment. Each capacitor is assigned a unique time slot in a switching sequence. The first level capacitor is used as a reference to which all other capacitors are sequentially compared. In the switching sequence, an AC signal is applied across each capacitor and compared with the inverted signal which is applied across the first level capacitor. A logic &#34;1&#34; results when the substance is present and a logic &#34;0&#34; when the substance is absent. The logic is decoded and converted to display the appropriate quantity of substance measured.

BACKGROUND OF THE INVENTION

The present invention relates to a digital fluid level sensing probesystem. A specific application involves liquid level sensing in ahostile environment such as the level of hot oil subjected to enginevibration in an automotive or other vehicle. State of the art in thecase of automotive vehicles is limited to provision of a low levelsignal visible on the dashboard with no intermediate level informationavailable other than by raising the hood and reading the level on a dipstick. This typically involves removing the dip stick, wiping it off,reinserting it, removing it for reading and reinserting it. The hostileenvironment precludes conventional liquid level sensing systems such asemployed for fuel gages and has called for a unique solution tomeasuring fluid levels, preferably to meet the following conditions:

(1) High temperature, high shock/vibration, volatile fluid, presence ofcontaminents.

(2) Low cost, mass producible, robust, repeatability and productuniformity.

(3) Universally applicable in any shape container, although specificallydesigned to accommodate the wide-short profile of the automobile oilpan.

(4) Small probe profile in terms of volume displacement, especially inprobe diameter.

(5) High resolution of volume measured per reading.

(6) Completely digital probe and electronics with data output directlyin Binary-Coded-Decimal format.

(7) Low power consumption.

The closest prior art, of which applicants are aware, to the systemdisclosed herein comprise three U.S. patents found in a preliminarysearch of the art.

Elis U.S. Pat. No. 3,935,739 discloses a capacitive probe with onecommon electrode and individual opposite electrodes. An AC signal isimpressed on the probe and simultaneously to all capacitive elements.The presence of dielectric material yields an analog current at theindividual capacitor which is greater than the current yield in theabsence of dielectric material. The AC current(s) are rectified andeither summed to drive an analog meter, or shaped and synchronized todrive a digital display. Inherent limitations result from the followingcharacteristics: the system measures absolute signal strength and istherefore subject to variations from temperature, Contaminants, anddielectric constants; nonuniformity of capacitance values among theindividual segments may cause inaccuracies; relatively large sizerequirements limit applications involving small containers.

Johnston U.S. Pat. No. 3,343,415 discloses a cylindrical capacitorcomprising a common electrode (inner) with individual electrodes (outer)spaced along a vertical (longitudinal) axis. Thee detection methodologycompares the signal output from any two adjacent capacitors. If bothcapacitors are immersed or if both capacitors are above the liquidlevel, the output signals are the same. If one capacitor is immersed andthe other is above the liquid level, the outputs differ by the effect ofthe dielectric constant of the liquid. Each capacitor is assigned aunique level and therefore the level of the fluid can be determined bythe detection of a difference signal. As in the Elis reference,nonuniformity of capacitance values among the individual segements maycause inaccuracies and relatively large size requirements limitapplications as to container size. The system uses a differentialcomparator method but the electronic implementation may involveinaccuracies due to isolating devices in series with the data signal.

Johnston U.S. Pat. No. 3,552,209 discloses a specific application of thecapacitive probe employed in the Johnston U.S. Pat. No. 3,343,415, whichdeals with a condition for measuring dynamic levels such as ocean swellsand tidal effects. It involves the use of sampling techniques wherebyliquid levels are instantaneously converted to pulse trains where thenumber of pulses in the train is a function of the liquid level and thetime interval over which the sample is taken. The pulse count isaveraged over a period of time and the mean level is determined. Thissystem eliminates anomolies (infrequent disturbances) by averaging theireffects.

.Iadd.Pope U.S. Pat. No. 4,589,077 discloses a multi-segment capacitiveprobe that includes a column of capacitors extending through a liquidlevel interface. Each capacitor of the probe is sequentially measured inresponse to microprocessor controlled instructions. A common electrodeis employed serving as one half of all probe capacitors. Similartechniques are also disclosed in Bristol U.S. Pat. No. 4,295,370 andMatsumara et al U.S. Pat. No. 4,434,657. .Iaddend.

GENERAL DESCRIPTION OF THE PRESENT INVENTION

The digital probe is constructed of two concentric cylinders, forming aset of discrete, cylindrical capacitors that use the substance whoselevel is to be measured as a dielectric material. The set of capacitorsis arranged along the axis of measurement where each capacitorrepresents a discrete level increment.

Each capacitor is assigned a unique time slot in a switching sequence.The first level capacitor is used as a reference to which all othercapacitors are sequentially compared. In the switching sequence, an ACsignal is applied across each capacitor and compared with the invertedsignal which is applied across the first level capacitor. A logic "1"results when the substance is present and a logic "0" when the substanceis absent. The level data is decoded and converted to display theappropriate quantity of substance measured.

As compared to known prior art, the digital level sensing system of thepresent disclosure includes a number of distinguishing features:

Flexible printed circuit capacitors and circuit connections are etchedonto a flexible printed circuit medium. The printed circuits areattached to rigid concentric cylinders. Each capacitor is formed byaligning the etched circuits or bands in concentricity along thelongitudinal axis of the probe. The rigid cylinders are positivelydetented to accurately align the printed circuits and therefore thecapacitor electrodes. The advantages of this implementation include massproducibility with no assembly of capacitors or wires or interconnects.It is possible to hold very tight tolerances with high accuracy,productivity and repeatability of capacitors. The probe is impervious toshock/vibration.

Individual isolated capacitor elements have no common electrodes orinterconnections. The electronic methodology can ground all capacitorsin the set except the reference and the single capacitor clement beingmonitored. This implementation eliminates inaccuracies of capacitancevalues due to effects from adjacent capacitors and/or from a commonelectrode.

Time sequenced methodology is employed wherein each capacitor element inthe set is assigned a unique time slot during which it is monitored.During the time slot, the selected capacitor clement is compared to areference capacitor element. All other capacitor elements may begrounded during the monitoring. The capacitor elements, are sequentiallymonitored in a wraparound mode and the output from each monitoredsequence is presented sequentially. The serial output of the sensedinformation minimizes the interconnection to the display unit. No morethan three wires are required and theoretically one wire with chassis asa return may be employed. Accuracy is greatly enhanced by isolating themonitored capacitor from extraneous circuit events. The output is adirect reading in Binary-Coded-Decimal format.

An integrated EPROM or other nonvolatile storage device is employed forthe container profile. A software algorithm for each container shape iswritten to a nonvolatile storage device. The memory contains volumetricdata, such as, incremental volume per incremental capacitor element,which is unique to that container shape or form factor. The nonvolatilestorage device can also contain other pertinent information, such as ID.Accordingly, the probe and electronics can be made universallyapplicable, independent of the container, and a single system can bemanufactured greatly enhancing inventory control and economics of scale.

The system provides small capacitance sensing ability using linearoperational amplifiers as current-to-voltage converters. The operationalamplifiers amplify the output data at the same time, thereby eliminatingthe voltage drop of resistors used in other systems which attenuate thedata and create inaccuracies. The system also eliminates the need forisolation diodes and other switching devices which attenuate the dataand create inaccuracies.

The foregoing distinguishing features combine to make achievable adigital sensing system which can have the smallest probe possible andstill be applicable in any larger configuration. It is possible toconstruct capacitances of less than one picofarad uniformly throughoutthe capacitor set and to detect the very small currents resulting, allin a digital mode, which allow the manufacture of small diameter probeswith the highest probe resolution (highest number of individual segmentsper unit height).

Accordingly, the present invention uniquely solves the problem ofmeasuring volume increments of a viscous fluid within a space having alarge surface area to depth ratio. An automobile oil pan is one suchapplication. The oil pan is shallow (less than 6 inches in usable depth)with a relatively larger surface area (greater than 120 square inches)containing a viscous fluid. Additionally, there are other challengesfrom particulate, high temperatures, shock, vibration and the presenceof volatile gases. The absolute volume of the automobile oil pan alsopresents a special condition by requiring that the measuring system besmall enough not to contaminate the measurement by displacement. Insummary, the disclosed system provides a means to measure smallvolumetric changes in fluid levels digitally in containers having theimpractical form factor of a large surface area to depth ratio and fromwhich the container volume is small and the fluid is viscous and forwhich the enviroment is hostile.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram identifying the major components of thesubject digital level sensing probe system;

FIG. 2 is a more detailed, schematic diagram identifying respectiveadditional elements employed;

FIGS. 3, 3A and 3B are schematic diagrams of a probe employed in thedisclosed embodiment with related capacitor formulae;

FIG. 4 is a further, still more detailed schematic drawing of thedigital level sensing system;

FIGS. 5A-5F are Constructional views of the probe tubular housing andend cap components thereof;

FIGS. 6A and 6B are layout views of flexible printed circuits shown asetched in the flat;

FIG. 6C is a sectional side elevation illustrating respective inner andouter flexible printed circuits as installed in the tubular housing ofthe probe assembly;

FIGS. 7A and 7B are layout views of flexible printed circuits for a amodified probe;

FIG. 7C is a sectional side elevation of the modified probe assembly;

FIG. 8 is an enlarged side elevation of the encasing tube per se of theFIGS. 7A-7C modification;

FIG. 9 is a partially sectioned side elevation of the outer mandrel perse of the FIGS. 7A-7C modification;

FIG. 10 is a partially sectioned side elevation of the inner mandrel perse of the FIGS. 7A-7C modification; and

FIG. 11 is a schematic view illustrating the sequence of digital levelsensing employed in the disclosed embodiment.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

With reference to FIG. 1, the main components of the system includeDIGITAL PROBE, IMBEDDED CONNECTOR, PROBE ASSOCIATED ELECTRONICS, REMOTECABLE, AND REMOTE DATA ELECTRONICS leading TO MAIN LOGIC OR DISPLAY, asillustrated in the block diagram.

With reference to FIGS. 2, 3 and 4, the main components of FIG. 1 aremore particularly identified by the schematic illustration of theDIGITAL PROBE A1 INNER CYLINDER and OUTER CYLINDER shown in an OILCONTAINER disposed over an IMBEDDED CONNECTOR; PROBE ASSOCIATEDELECTRONICS including EXCITATION "EX" with schematic blocks B1, C, D andE, representing respectively a LOW Z SWITCHING NETWORK, a SINUSOIDALOSCILLATOR, an AMPLIFIER and an INVERTER; POWER SUPPLY "P1" having +5VDC, -5 VDC and +8 VDC, RETURN "RE" with schematic blocks B2, F, G andH, representing respectively, a HIGH Z SWITCHING NETWORK, aCURRENT/VOLTAGE CONVERTER/AMPLIFIER, an AMPLIFIER and a SCHMITT TRIGGERAND FILTER; and LOGIC CONTROL "LC" with schematic blocks 11, J1, K andL, representing respectively SWITCHES CONTROL LATCH, a BINARY COUNTER, aSYSTEM CLOCK and a LOGIC CONTROL CIRCUIT.

REMOTE CABLE connections are provided from PROBE ASSOCIATED ELECTRONICSto REMOTE DATA ASSOCIATED ELECTRONICS which include: POWER SUPPLY "P2"WITH +5 VDC; REMOTE LOGIC "IC", with schematic blocks J2 and I2,representing respectively a DECADE COUNTER, and REMOTE INTERFACE LATCH;and a block "M" representing a LEVEL-TO-VOLUME CONVERSION leading to anAUTO MAIN LOGIC OR DISPLAY "N".

With further reference to FIG. 3, a fixed capacitor A2, is providedbetween the EXCITATION, B1 LOW Z SWITCHING NETWORK and RETURN, B2, HIGHZ SWITCHING NETWORK; an INVERTED EXCITATION capacitor is connected toRETURN; and individual capacitors 2-16 are provided between respectiveNETWORKS B1 and B2.

The capacitance of each subsensor element is determined by its height L,outer radius R1, inner radius R2, and dielectric constant, in accordancewith the general formula shown, with capacitancc C^(F) equal to that ofa subsensor full of the measured substance and capacitance C_(E), thatof a subsensor absent the measured substance.

With reference to FIGS. 5A-5F, aluminum probe housing 10 is formed withgap 11 and is adapted for insertion of flexible printed circuits withetched electrodes formed into tubes which are retained by respective endcaps, FIGS. 5C-5F, nested within aluminum housing 10. Such end caps areshown enlarged, relative to outer housing 10, and are assembled toretain the flexible printed circuits as follows: Lower end cap 13 nestswithin annular recess 14 in lower end cap 15 which in turn fits withinthe lower end of housing 10. Upper end cap 16 nests within recess 17 inupper end cap 18 which in turn fits within the upper end of housing 10.An inner flexible cylindrical printed circuit, not shown in FIGS. 5A-5F,is retained within inner walls 19 and 20 of respective end caps 13 and16 and an outer cylindrical flexible printed circuit is retained betweenthe outer perimeters 21 and 22 of end caps 13 and 16 and innerperimeters 23 and 24 of end caps 15 and 18. The assembled relation ofprobe components is illustrated in FIG. 6C wherein outer printed circuit25, shown as etched in the flat in FIG. 6A, is formed into a tube 25a asshown in FIG. 6C. Likewise, inner printed circuit 26 of FIG. 6B isformed into tube 26a in the assembled probe.

With reference to FIGS. 7A-7C, 8, 9 and 10, a modified probe employs apair of rigid spools onto the cylindrical surfaces of which inner andouter flexible circuits are adhered in facing relation. Accordingly,outer printed circuit 27, shown in the flat in FIG. 7A, is adhered tothe inner surface of outer mandrel (spool) 28 nested within encasingtube 29 and inner flexible printed circuit 30, shown in the flat in FIG.7B, is adhered to the outer surface of inner mandrel (spool) 31. Therespective spool ends are sized to nest respectively within encasingtube 29 while providing appropriate space 32 between the facing printedcircuits for dielectric material to be detected.

With reference to FIG. 11, in summary, the herein disclosed digitallevel sensing probe system has the following characteristics:

The digital sensor is a sensor which senses only discrete values of thesensed property by means of a yes ("1") or no ("0") output for eachmonitored division. This is in contrast to analog sensors which give acontinuous output of the sensed property value.

The digital sensor is an array of subsensors, each of which reads "1" or"0", or ON/OFF, or binary format, only. The number of subsensors in thearray, or the separation between two adjacent divisions to be read bythe sensor, determines the resolution of the measurement. The discretesubsensors are controlled by a logic circuit which rationalizes themeasured value.

The capacitive type digital level-sensing probe is constructed of anarray of discrete, cylindrical, equal capacitors that use the substancewhose level is to be measured as the dielectric material. The array ofcapacitors is arranged along the axis of measurement, where eachcapacitor represents a discrete level increment.

The control logic circuit controls the probe capacitors (the set ofsubsensors) by a switching network. Each capacitor is assigned a uniquetime slot in a switching sequence. The electronics allows for thegrounding of all other capacitor elements during this time. The first(bottom) level capacitor is used as a reference to which all othercapacitors are sequentially compared. This method has the advantage ofcompensating for variation in the dielectric constant of the measuredsubstance and for changes in the probe itself. In the switchingsequence, a signal V=V_(o) * SIN(wt) is applied across each capacitorstarting from the second element (subsensor) above the referenceelement. Each application of signal V applied to one subsensor resultsin a current output signal I=C*dV/dt=C. *V_(o) *wCOS(wi). This currentsummed with the current outputting from the reference element which, inturn, has been excited by an inverted signal V_(i) =V_(o) *SIN(wt+ρ).Hence, when the substance is present in the sequenced capacitor, theresult is a null, or zero current (assuming the reference element isimmersed in substance) which translates to a logic "1". When thesubstance is absent in the sequenced segment, but present in thereference segment, the resulting summed current is non-zero, translatinginto a logic "0".

When the first logic "0" is obtained, the control circuit electronicallytags the substance level and instructs the counter controlling theswitching sequence to reset and start the sequence over. The completelyfull (of substance) condition is determined by obtaining the logic "0"when the first (reference) element is compared to a fixed element (A2 inthe schematics), the value of which is different enough from the "full"capacitance value, C_(F). In the completely empty condition (referenceelement is empty), level zero is determined by choosing the fixedcapacitor value to be close to the "empty" capacitance value, C_(E),then the level zero is recognized through the counter and not by a logic"0" from the probe. The acknowledged level in the logic circuit (abinary number) is decoded and converted to display the appropriatequantity of substance measured.

As shown in FIG. 11, a reading of level 4 means that the substance levelis between 3.5 and 4.5 increment levels from the bottom increment.

As can be seen from the above description, the capacitor subsensorelements are critical to the operation of the digital probe. In order tosatisfy the accuracy, resolution, and probe size criteria, the capacitorelements meet three conflicting, and heretofore mutually exclusiverequirements: Very small physical size of the electrodes in order to geta sufficient number of vertical elements to achieve resolution and asmall probe diameter; leave enough space between electrodes to allowunrestricted draining of viscous liquid; maintain dimensional tolerancesof the capacitor electrode areas and gap distance to achieve accuratecapacitance values; insure that all capacitors are uniform in terms ofcapacitance values.

The printed circuit discrete capacitors, having independence from commonelectrodes or interconnects, afford the ability to make very small, veryclose toleranced capacitor electrodes on a mass production scale. Thetime sequencing and isolation of capacitors as well as the currentsumming methodology allow work with the very small capacitancesresulting.

While in the preferred embodiment the capacitors are constructed byprinted circuit etch on flexible material, it would be possible to makesuch capacitors as an integral part of rigid cylinders or couldthemselves be the cylinders, while retaining the functional performanceof the preferred embodiment. Also, while the disclosed constructionemploying concentric cylinders is preferred for high accuracyrequirements, other printed circuit configurations are possible, e.g.,the circuits shown as printed in the flat in FIGS. 6A and 6B, or inFIGS. 7A and 7B, could be mounted in facing relation on flat supportelements with appropriate aligned spacing.

We claim:
 1. Digital fluid level sensing probe system comprising digitalprobe fluid level sensing means constructed of a set of .Iadd.isolated.Iaddend.capacitors arranged along an axis of measurement where eachcapacitor represent a discrete level increment in dielectric materialfluid to be measured, switching sequence means wherein each capacitor isassigned a unique time slot for sequential comparison .Iadd.to a.Iaddend.reference .[.to a first level capacitor.]., means for applyingan AC signal in switching sequence across .[.each capacitor above thefirst level capacitor.]. .Iadd.succeeding capacitors.Iaddend., means for.[.applying an inverted AC signal across the first level capacitor.]..Iadd.establishing a reference signal.Iaddend.for comparison with eachof said AC signals applied in switching sequence, including means fordiscriminating between .Iadd.the .Iaddend.respective .Iadd.compared.Iaddend.signals to provide a logic "1" when fluid is present and alogic "0" when fluid is absent, or vice versa, and means for decodingand converting said logic to display the appropriate quantity of fluidmeasured in increments corresponding to said .[.discrete.]..Iadd.isolated .Iaddend.capacitors.
 2. System of claim 1 wherein eachcapacitor includes flexible printed circuits with capacitor electrodesand circuit connections etched onto a flexible printed circuit medium.3. System of claim 2 wherein said flexible printed circuit capacitorsare attached to rigid concentric cylinders.
 4. System of claim 2 whereineach capacitor is formed by aligning the etched circuits inconcentricity along the longitudinal axis of the probe.
 5. System ofclaim 3 wherein said rigid cylinders are positively detented toaccurately align the printed circuits and therefore the capacitorelectrodes.
 6. System of claim 1 wherein said capacitors are constructedas an integral part of rigid cylinders.
 7. System of claim 1 whereinsaid Capacitors have no common electrodes or interconnections.
 8. Systemof claim 1 wherein all capacitors in the set .Iadd.to which said ACsignal is applied in switching sequence .Iaddend.are grounded except the.[.first level capacitor and the.]. single capacitor .[.beingmonitored.]. .Iadd.having the AC signal applied.Iaddend..
 9. System ofclaim 1 including reset means whereby each capacitor in the set isrepeatedly recompared while being .[.monitored.]. .Iadd.compared.Iaddend.in its assigned unique time slot.
 10. System of claim 9.[.including reset means.]. wherein the time slots when each capacitoris compared to the .[.first level capacitor.]. .Iadd.reference.Iaddend.are reset .Iadd.by said reset means .Iaddend.each time a signalthat fluid is absent occurs.
 11. System of claim 10 including means forgrounding all .[.other.]. capacitors .[.during the monitoring.]..Iadd.in the set to which said AC signal is applied in switchingsequence except the single capacitor having the AC signal applied..Iaddend.
 12. System of claim 1 including means wherein the capacitorsare sequentially monitored in a wraparound mode.
 13. System of claim 12including means for applying a signal V across each capacitor insequence wherein the output from each monitored sequence is presentedsequentially.
 14. System of claim 13 including means for reading theoutput directly in Binary-Coded-Decimal format.
 15. System of claim 1including an integrated EPROM or equivalent nonvolatile storage devicefor container profile.
 16. System of claim 15 including a softwarealgorithm for each container shape written into the nonvolatile storagedevice.
 17. System of claim 16 wherein said nonvolatile storage deviceincludes memory for volumetric data.
 18. System of claim 1 includinglinear operational amplifiers as current-to-voltage converters. 19.System of claim 18 wherein said operational amplifiers include means foramplifying the output data at the same time.
 20. System of claim 1including means for providing said set of capacitors in a uniform set ofindividual capacitors having capacitances of less than one picofaradeach.